Countercurrent multistage fluidized bed reactor for immobilized biocatalysts: I. Modeling and simulation

For the application of immobilized enzymes, fixed bed reactors are used almost exclusively. Fixed bed reactors have specific disadvantages, especially for processes with a deactivating catalyst. Therefore, we have studied a novel reactor type with continuous transport of the immobilized biocatalyst. Flow of biocatalyst is countercurrent to the substrate solution. Because of a stagewise reactor design, back-mixing of biocatalyst is very limited and transport is nearly plug flow. The reactor operates at a constant flow rate and conversion, due to constant holdup of catalytic activity. The reactor performance is compared with a configuration of fixed bed reactors. For reactions in the first-order regime, enzyme requirements in this new reactor are slightly less than for fixed bed processes. The multistage fluidized bed appears to be an attractive reactor design to use biocatalyst to a low residual activity. However, nonuniformity of the particles might affect plug flow transport of the biocatalyst. A laboratory scale reactor and experiments are described in Part II(1) of this series. Hydrodynamic design aspects of a multistage fluidized bed are discussed in more detail in Part III.(2).     

Dynamic mathematical modeling of sulfate reducing gas-lift reactors

This paper presents a mathematical model able to simulate under dynamic conditions the physical, chemical and biological processes prevailing in a biological sulfate reducing gas-lift reactor. The proposed model is based on differential mass balance equations for substrates, products and bacterial groups involved in a sulfate reduction process. Heterotrophic sulfate reducing bacteria (HSRB), autotrophic sulfate reducing bacteria (ASRB), homoacetogenic bacteria (HB), methanogenic archaea (MA) and acetate degraders (AD) are the microbial groups taken into account in the model. The model is also used to validate a steady-state design model previously proposed by Esposito et al. [1].The proposed model is able to simulate the competition between the biological bacteria growing in the reactor, and predict the performance of a gas-lift reactor. The model includes two main parts: (1) a kinetic part including growth, metabolism and competition of SRB, HB, MA and AD in the system and (2) a mass-transfer part describing the thermodynamic concentration equilibria of gaseous components in the liquid and gas phase. The model has been validated using experimental data obtained by operating a laboratory-scale gas-lift reactor as described in Esposito et al. [2].The model can be applied to simulate the sulfate reduction process in a gas-lift reactor for several purposes, such as the evaluation of the optimal process conditions in terms of COD:SO₄^2? ratio, hydraulic retention time and gas input flow. In particular, model simulations reported in this paper show the model capability to predict the prevailing bacterial species and concentrations in the reactor as a function of the hydraulic retention time.     

升流厌氧污泥层反应器动力学模型

用碘离子作示踪剂,采用矩形脉冲示踪法测定升流厌氧污泥层(UASB)反应器的流动分布。建立了申级返混加沟流模型。模型简单,能够反映反应器流动分布,具有较强的拟合能力和良好的适用性。运用流动模型和Monod方程,建立了UASB反应器稳态模型,并对模型参数进行了估计。通过灵敏度分析,进水基质浓度S。,废水流量Q,最大比基质降解速率,μmax 对出水基质浓度有较大影响。在稳态模型的基础上又建立了UASB反应器动态模态,利用此模型,对出水基质浓度序列Se,和产气量序列Qg进行计算预测,平均偏差分别5．40％和7．46％,标准偏差分别为7．02％和9．66％。     

Predicting the performance of immobilized enzyme reactors using reversible Michaelis–Menten kinetics

A general mathematical model is developed in the present work for predicting the steady state performance of immobilized enzyme reactor performing reversible Michaelis - Menten kinetics. The model takes into account the effect of external diffusional limitations, the axial dispersion and the equilibrium constant on reactor performance quantified as relative substrate conversion and yield. The performance of reactor is characterized using the dimensionless parameters of Damkohler number, Stanton number, Peclet number, the equilibrium constant and the dimensionless input substrate concentration. The reactor performance is described for the two extreme cases of plug flow reactor (PFR) and continuous stirred tank reactor (CSTR) in addition to the intermediate case of dispersed plug flow reactor (DPFR). The performance of reactor is compared for the two cases of zero order and reversible first order kinetics.     

Continuous coagulation of milk using immobilized enzymes in a fluidized-bed reactor

Milk-clotting enzymes such as pepsin, chymosin, chymotrypsin, and M. miehei proteases were immobilized on porous, alkylamine glass and incorporated into a fluidized-bed continuous coagulation scheme. Only pepsin and calf rennet retained sufficient activity towards skim milk to warrant further studies. Comparison of kinetic data with fixed-bed reactors revealed the overall superior performance of fluidized beds; higher clotting activities were possible while avoiding plugging problems and high pressure drops common to fixed-bed reactors. Film diffusion and catalyst back-mixing appear to be significant factors in the overall kinetics. All enzymes lost activity on exposure to skim milk. The inactivation rates were lower at high substrate pH and insignificantly affected by reactor temperature. Nitrogen and sialic acid accumulation on the porous glass paralleled the loss in activity in the initial stages. Attempts to regenerate the immobilized enzymes were partially successful.     

An immobilized cell reactor with simultaneous product separation. I. Reactor design and analysis

A four-phase reactor-separator (gas, liquid, solid, and immobilized catalyst) is proposed for fermentations characterized by a volatile product and nonvolatile substrate.In this reactor, the biological catalyst is immobilized onto a solid column packing and contacted by the liquid containing the substrate.A gas phase is also moved through the column to strip the volatile product into the gas phase. The Immobilized Cell Reactor-Separator (ICRS) consists of two basic gas-liquid flow sections: a cocurrent "enricher" followed by a countercurrent-"stripper".In this article, an equilibrium stage model of the reactor is developed to determine the feasibility and important operational variables of such a reactor-separator. The ICRS concept is applied to the ethanol from whey lactose fermentation using some preliminary immobilized cell reactor performance data. A mathematical model for a steady-state population based on an adsorbed monolayer of cells is also developed for the reactor. The ICRS model demonstrated that the ICRS should give a significant increase in reactor productivity as compared to an identically sized Immobilized Cell Reactor (ICR) with no separation. The gas-phase separation of the product also allows fermentation of high inlet substrate concentrations. The model is used to determine the effects of reactor parameters on ICRS performance including temperature, pressure, gas flow rates, inlet substrate concentration, and degree of microbial product inhibition.     

Effect of ethanol, acetate, and phenol on toluene degradation activity and tod-lux expression in Pseudomonas putida TOD102: evaluation of the metabolic flux dilution model

The reporter strain Pseudomonas putida TOD102 (with a tod-lux fusion) was used in chemostat experiments with binary substrate mixtures to investigate the effect of potentially occurring cosubstrates on toluene degradation activity. Although toluene was simultaneously utilized with other cosubstrates, its metabolic flux (defined as the toluene utilization rate per cell) decreased with increasing influent concentrations of ethanol, acetate, or phenol. Three inhibitory mechanisms were considered to explain these trends: (1) repression of the tod gene (coding for toluene dioxygenase) by acetate and ethanol, which was quantified by a decrease in specific bioluminescence; (2) competitive inhibition of toluene dioxygenase by phenol; and (3) metabolic flux dilution (MFD) by all three cosubstrates. Based on experimental observations, MFD was modeled without any fitting parameters by assuming that the metabolic flux of a substrate in a mixture is proportional to its relative availability (expressed as a fraction of the influent total organic carbon). Thus, increasing concentrations of alternative carbon sources "dilute" the metabolic flux of toluene without necessarily repressing tod, as observed with phenol (a known tod inducer). For all cosubstrates, the MFD model slightly overpredicted the measured toluene metabolic flux. Incorporating catabolite repression (for experiments with acetate or ethanol) or competitive inhibition (for experiments with phenol) with independently obtained parameters resulted in more accurate fits of the observed decrease in toluene metabolic flux with increasing cosubstrate concentration. These results imply that alternative carbon sources (including inducers) are likely to hinder toluene utilization per unit cell, and that these effects can be accurately predicted with simple mathematical models.     

Sequential Transhydroxylations Converting Hydroxyhydroquinone to Phloroglucinol in the Strictly Anaerobic, Fermentative Bacterium Pelobacter massiliensis

The recently isolated fermenting bacterium Pelobacter massiliensis is the only strict anaerobe known to grow on hydroxyhydroquinone (1,2,4-trihydroxybenzene) as the sole source of carbon and energy, converting it to stoichiometric amounts of acetate. In this paper, we report on the enzymatic reactions involved in the conversion of hydroxyhydroquinone and pyrogallol (1,2,3-trihydroxybenzene) to phloroglucinol (1,3,5-trihydroxybenzene). Cell extracts of P. massiliensis transhydroxylate pyrogallol to phloroglucinol after addition of 1,2,3,5-tetrahydroxybenzene (1,2,3,5-TTHB) as cosubstrate in a reaction identical to that found earlier with Pelobacter acidigallici (A. Brune and B. Schink, J. Bacteriol. 172:1070-1076, 1990). Hydroxyhydroquinone conversion to phloroglucinol is initiated in cell extracts without an external addition of cosubstrates. It involves a minimum of three consecutive transhydroxylation reactions characterized by the transient accumulation of two different TTHB isomers. Chemical synthesis of the TTHB intermediates allowed the resolution of the distinct transhydroxylation steps in this sequence. In an initial transhydroxylation, the hydroxyl group in the 1-position of a molecule of hydroxyhydroquinone is transferred to the 5-position of another molecule of hydroxyhydroquinone to give 1,2,4,5-TTHB and resorcinol (1,3-dihydroxybenzene) as products. Following this disproportionation of hydroxyhydroquinone, the 1,2,4,5-isomer is converted to 1,2,3,5-TTHB, an enzymatic activity present only in hydroxyhydroquinone-grown cells. Finally, phloroglucinol is formed from 1,2,3,5-TTHB by transfer of the 2-hydroxyl group to either hydroxyhydroquinone or resorcinol. The resulting coproducts are again cosubstrates in earlier reactions of this sequence. From the spectrum of hydroxybenzenes transhydroxylated by the cell extracts, the minimum structural prerequisites that render a hydroxybenzene a hydroxyl donor or acceptor are deduced.     

Kinetics of escherichia coli hydrogen production during short term repeated aerobic-anaerobic fluctuations

A coupled reactor system, consisting of one aerated stirred reactor and one anaerobic plug flow reactor was constructed. Circulation of microbial cells in this system, is chosen to represent the insufficient mixing conditions, present in a large scale vessel. Hereby it is shown, that oscillating oxygen concentrations, give effects on E.coli metabolism. The hydrogen production, resulting from mixed acid fermentation of anaerobically grown cells, is used to indicate the metabolic changes. These changes are measured as hydrogen concentration in the gas outlet of the aerated fermenter, with a Pd-MOS sensor. The dependency of the hydrogen evolution on the anaerobic residence time is shown, and the relevance of this model system for studies on the bioreactor performance, are discussed.     

Vegetation patterns generated by a wind driven sand-vegetation system in arid and semi-arid areas

Wind has been proposed as a driving factor in determining vegetation patterns, but there are few dynamic models that include both vegetation and wind. In this paper, we present a dynamic model to investigate how a vegetation pattern is generated and affected by wind. In the model, the effects of prevailing wind and non-prevailing wind on sand and vegetation are modeled respectively as advection terms and diffusion terms. With these considerations and proper parameter values that satisfy Turing bifurcation conditions, labyrinth and banded vegetation patterns are obtained in two situations of wind. By changing wind transportation capacity, we simulate the adaptation process from one vegetation pattern to another. With environmental changes of large amplitude, the width of vegetation bands varies while the wavelength can increase but does not decrease in our simulation. Then we describe the difference between simulated patterns and real patterns. And in the discussion, we explain the mechanism that forming patterns and the consistency of this research with other studies.     

Prostaglandin H synthase. Effects of peroxidase cosubstrates on cyclooxygenase velocity

Many cosubstrates for the peroxidase activity of prostaglandin H synthase-1 (PGHS-1) have been reported to produce a large (2-7-fold) increase in the cyclooxygenase velocity in addition to a substantial increase in the number of cyclooxygenase catalytic turnovers. The large stimulation of cyclooxygenase velocity has become an important criterion for evaluation of putative PGHS reaction mechanisms. This criterion has been a major weakness of branched-chain tyrosyl radical mechanisms, which correctly predict many other cyclooxygenase characteristics. Our computer simulations based on a branched-chain mechanism indicated that the uncorrected oxygen electrode signals commonly used to monitor activity can seriously overestimate the effects of cosubstrate on cyclooxygenase velocity. The simulation results prompted re-examination of the effect of several cosubstrates (phenol, acetaminophen, N,N,N',N'-tetramethylphenylenediamine, and Trolox) on PGHS-1 cyclooxygenase velocity. Cyclooxygenase kinetics were examined at reduced temperature or elevated pH, where the oxygen electrode signal can be corrected to provide reliable oxygen consumption trajectories. The cosubstrates produced only a slight (10-60%) stimulation of the cyclooxygenase velocity. Peroxidase cosubstrates thus have a much smaller stimulatory effect on cyclooxygenase velocity than previously reported. This corrects a longstanding misperception of cosubstrate effects, provides more realistic kinetic constraints on PGHS mechanisms, and removes what was a major deficiency of branched-chain tyrosyl radical mechanisms.     

A Modeling and Simulation Study of the Role of Suspended Microbial Populations in Nitrification in a Biofilm Reactor

Many biological wastewater treatment processes are based on bacterial biofilms, i.e. layered aggregates of microbial populations deposited on surfaces. Detachment and (re-)attachment leads to an exchange of biomass between the biofilm and the surrounding aqueous phase. Traditionally, mathematical models of biofilm processes do not take the contribution of the suspended, non-attached bacteria into account, implicitly assuming that these are negligible due to the relatively small amount of suspended biomass compared to biofilm biomass. In this paper, we present a model for a nitrifying biofilm reactor that explicitly includes both types of biomass. The model is derived by coupling a reactor mass balance for suspended populations and substrates with a full one-dimensional Wanner–Gujer type biofilm model. The complexity of this model, both with respect to mathematical structure and number of parameters, prevents a rigorous analysis of its dynamics, wherefore we study the model numerically. Our investigations show that suspended biomass needs to be considered explicitly in the model if the interests of the study are the details of the nitrification process and its intermediate steps and compounds. However, suspended biomass may be neglected if the primary interests are the overall reactor performance criteria, such as removal rates. Furthermore, it can be expected that changes in the biofilm area, attachment, detachment, and dilution rates are more likely to affect the variables primarily associated with the second step of nitrification, while the variables associated with the first step tend to be more robust.     

Theoretical analysis of G6P production and simultaneous ATP regeneration by conjugated enzymes in an ultrafiltration hollow-fiber reactor

The performance of an ultrafiltration hollow-fiber reactor, in which the enzymatic synthesis of glucose 6-phosphate from glucose and cofactor ATP and the enzymatic regeneration of ATP from ADP and acetyl phosphate are performed simultaneously, was analyzed theoretically. A simple analytical model in which the liquid flowing in the fiber tubes is assumed to be plug flow, and the radial concentration gradients in the tube and shell sides are both neglected, could simulate the reactor performance with satisfactory accuracy. The simulation elucidated the effects of the reactor configurations and various operational conditions on glucose conversion, ATP recycle number, and space-time yield. If the fiber tubes, through which the permeability of the relevant components such as substrates is high, were packed as much as possible in the reactor, good reactor performance could be expected. Furthermore, with a sufficiently high enzyme concentration, low ATP concentration in the feed solution, and appropriate space velocity, good space-time yield with high glucose conversion and with very high ATP recycle number is theoretically possible.     

Performance of the continuous glucose isomerase reactor system for the production of fructose syrup

Glucose isomerase in the form of heat-treated whole-cell enzyme prepared from Streptomyces phaeochromogenus follows the reversible single-substrate reaction kinetics in isomerization of glucose to fructose. Based on the Kinetic constants determined and the mathematical model of the reactor system developed, the preformance of a plug-flow-type continuous-enzyme reactor system was studied experimentally and also simulated with the aid of a computer for the ultimate objective of optimization of the glucose isomerase reactor system. The enzyme decay function for both the enzyme storage and during the use in the continuous reactor, was found to follow the first-order decay kinetics. When the enzyme decay function is taken into consideration, the ideal homogeneous enzyme reactor kinetics provided a satisfactory working model without further complicatin of the mathematical model, and the results of computer simulation were found to be in good agreement with the experimental results. Under a given set of constraints the performance of the continuous glucose isomerase reactor system can be predicted by using the computer simulation method described in this paper. The important parameters studied for the optimization of reactor operation were enzyme loading, mean space time of the reactor, substrate feed concentration, enzyme decay constants, and the fractional conversion, in addition to the kinetic constants. All these parameters have significant effect on the productivity. Some unique properties of the glucose isomerization reaction and its effects on the performance of the continuous glucose isomerase reactor system have been studied and discussed. The reaction kinetics of glucose isomerase and the effects of both the enzyme loading and the changes in reaction rate within a continuous reactor on the productivity are all found to be of particular importance to this enzyme reactor system.     

Effect of mixing on the washout and steady-state performance of continuous cultures

The effects of mixing on the critical mean holding time for washout and the steady state performance of growth processes in continuous flow reactors are investigated. Macromixing, micromixing, and cell recycle arc considered. The tanks-in-series model composed of N completely mixed flow reactors, the dispersion model, the plug flow model, and a combined model composed of a plug flow reactor and a continuous stirred tank flow reactor connected in series arc used to represent the macro-mixing or residence time distribution. The extreme cases of micromixing, namely, complete segregation and maximum mixedness, as well as intermediate states of micromixing are investigated to determine their effects on washout and the occurence of multiple steady states. A technique for predicting the maximum mixedness washout condition from a knowledge of the residence time distribution is presented and used to determine the washout condition for the dispersion model under maximum mixedness conditions.     

Empirical model for the autotrophic biodegradation of thiocyanate in an activated sludge reactor

AIMS: The aim of this investigation was to develop an empirical model for the autotrophic biodegradation of thiocyanate using an activated sludge reactor. METHODS AND RESULTS: The methods used for this purpose included the use of a laboratory scale activated sludge reactor unit using thiocyante feed concentrations from 200 to 550 mg x l(-1). Reactor effluent concentrations of <1 mg x l(-1) thiocyanate were consistently achieved for the entire duration of the investigation at a hydraulic retention time of 8 h, solids (biomass) retention of 18 h and biomass (dry weight) concentrations ranging from 2 to 4 g x l(-1). A biomass specific degradation rate factor was used to relate thiocyanate degradation in the reactor to the prevailing biomass and thiocyanate feed concentrations. A maximum biomass specific degradation rate of 16 mg(-1) x g(-1) x h(-1) (mg thiocyanate consumed per gram biomass per hour) was achieved at a thiocyanate feed concentration of 550 mg x l(-1). The overall yield coefficient was found to be 0.086 (biomass dry weight produced per mass of thiocyanate consumed). CONCLUSION: Using the results generated by this investigation, an empirical model was developed, based on thiocyanate feed concentration and reactor biomass concentration, to calculate the required absolute hydraulic retention time at which a single-stage continuously stirred tank activated sludge reactor could be operated in order to achieve an effluent concentration of <1 mg x l(-1). The use of an empirical model rather than a mechanistic-based kinetic model was proposed due to the low prevailing thiocyanate concentrations in the reactor. SIGNIFICANCE AND IMPACT OF THE STUDY: These results represent the first empirical model, based on a comprehensive data set, that could be used for the design of thiocyanate-degrading activated sludge systems.     

Modeling, simulation, and optimization of bacterial leaching reactors

Bacterial leaching represents an unusual problem in biochemical engineering, because the substrate for bacterial growth is not supplied directly, but is a product of another reaction, the leaching of mineral particles. In addition, leaching is a heterogeneous reaction dependent on the particle-size distribution in the feed and on the kinetics of particle shrinkage. In this study, these effects are incorporated in the material balance for each mineral by the number balance. Examination of the number balance gives rise to a novel analysis of the competing technologies for leaching. The model is completed by the addition of material balances for the ferrous and ferric ions, the dissolved oxygen, and for each bacterial species to the number balance for each mineral present in the feed. The model is compared with pilot plant data for three different ores. It is shown that the model is in excellent agreement with the data. The performance of a bacterial leaching reactor is explored using the model, and the washout and sensitivity criteria are determined. It is shown that there are three washout conditions, in which the leaching conversion drops to zero. The washout conditions are dependent on the growth rate of the bacteria, on the rate of dissolution of the mineral, and on the rate of mass transfer of oxygen to the reactor. The critical washout condition is that arising from the rate of mineral dissolution. The optimization of a plant in which continuous tank reactors are configured in series is addressed. This analysis shows that the primary reactor should be between 1.5 and 2 times the size of each of the secondary reactors in a series combination.     

Controlled shear affinity filtration (CSAF): a new technology for integration of cell separation and protein isolation from mammalian cell cultures

Up-flow anaerobic sludge blanket (UASB) reactors are being used with increasing regularity all over the world, especially in India, for a variety of wastewater treatment operations. Consequently, there is a need to develop methodologies enabling one to determine UASB reactor performance, not only for designing more efficient UASB reactors but also for predicting the performance of existing reactors under various conditions of influent wastewater flows and characteristics. This work explores the feasibility of application of an artificial neural network-based model for simulating the performance of an existing UASB reactor. Accordingly, a neural network model was designed and trained to predict the steady-state performance of a UASB reactor treating high-strength (unrefined sugar based) wastewater. The model inputs were organic loading rate, hydraulic retention time, and influent bicarbonate alkalinity. The output variables were one or more of the following, effluent substrate concentration (Se), reactor bicarbonate alkalinity, reactor pH, reactor volatile fatty acid concentration, average gas production rate, and percent methane content of the gas. Training of the neural network model was achieved using a large amount of experimentally obtained reactor performance data from the reactor mentioned above as the training set. Training was followed by validation using independent sets of performance data obtained from the same UASB reactor. Subsequently, simulations were performed using the validated neural network model to determine the impact of changes in parameters like influent chemical oxygen demand (COD) concentration and hydraulic retention time on the reactor performance. Simulation results thus obtained were carefully analyzed based on qualitative understanding of UASB process and were found to provide important insights into key variables that were responsible for influencing the working of the UASB reactor under varying input conditions.     

Mixing and phase hold-ups variations due to gas production in anaerobic fluidized-bed digesters: influence on reactor performance

The influence of mixing and phase hold-ups on gas-producing fluidized-bed reactors was investigated and compared with an ideal flow reactor performance (CSTR). The liquid flow in the anaerobic fluidized bed reactor could be described by the classical axially dispersed plug flow model according to measurements of residence time distribution. Gas effervescence in the fluidized bed was responsible for bed contraction and for important gas hold-up, which reduced the contact time between the liquid and the bioparticles. These results were used to support the modeling of large-scale fluidized-bed reactors. The biological kinetics were determined on a 180-L reactor treating wine distillery wastewater where the overall total organic carbon uptake velocity could be described by a Monod model. The outlet concentration and the concentration profile in the reactor appeared to be greatly influenced by hydrodynamic limitations. The biogas effervescence modifies the mixing characteristics and the phase hold-ups. Bed contraction and gas hold-up data are reported and correlated with liquid and gas velocities. It is shown that the reactor performance can be affected by 10% to 15%, depending on the mode of operation and recycle ratio used. At high organic loading rates, reactor performance is particularly sensitive to gas effervescence effects. Copyright 1998 John Wiley & Sons, Inc.     

Factor analysis of transesterification reaction of waste oil for biodiesel production

In the present paper a factor analysis is presented for the enzymatic transesterification of waste oil for biodiesel production. The experimental data on batch reactor evidence two key variables: enzyme loading and mixing conditions. These variables were subjected to a factor analysis and their combined effect on the reaction performance was determined. Response surface methodology (RSM) was used based on a linear first order model (steepest ascent method) and on a second order one in proximity of the optimal solution. The result was a model able to predict reaction performance within the range of mixing rates and enzyme amount considered for model formulation and outside of it, as shown in the final validation. Best performances were obtained at high stirring and high enzyme loading.